1. Field of the Invention
The field of the invention concerns a frequency-doubled laser, and in particular a method and apparatus for generating a frequency doubled beam using Type II phase-matching in an intracavity second harmonic generation crystal.
2. Description of the Prior Art
Second Harmonic Generation (SHG) provides a means of doubling the frequency of a laser source. In this process, a fundamental electromagnetic wave in a nonlinear medium induces a polarization wave with a frequency that is double that of the fundamental wave. Because of dispersion in the refractive index of the medium, the phase velocity of such a wave is a function of its frequency, so the phase of the induced second harmonic polarization wave is retarded from that of the fundamental wave. Since the vector sum of all the generated second harmonic polarizations yields the SHG intensity, the intensity is limited by the phase retardation. A technique, known as phase matching, is designed to overcome this difficulty by utilizing in uniaxial and biaxial crystals the natural birefringence, i.e. the difference in the phase velocity as a function of polarization, to offset the dispersion effect so that the fundamental and second harmonic wave can propagate in phase.
There are two well known types of phase matching, which empoly the polarization vectors of the incident fundamental wave in different ways.
In Type I phase matching, the fundamental wave is polarized perpendicular to the crystal's optic axis (an O or ordinary ray) and the induced Second Harmonic wave is polarized parallel to the optical axis (an E or extraordinary ray). (A method utilizing Type I phase matching is described in U.S. Pat. No. 4,413,342.) Since the fundamental wave is polarized along the optic axes of the crystal, there is no change in its linear polarization when it exits from the crystal. An intracavity Type I SHG arrangement can easily be adopted to take advantage of the higher power density available within the laser cavity because the introduction of the SHG crystal will not produce a significant polarization loss.
In Type II phase matching, the linearly polarized fundamental wave is equally divided into O and E rays by requiring its polarization to be 45.degree. with respect to the optic axis of the crystal; the output second harmonic wave which results is linearly polarized parallel to the optic axis (an E ray). Here, the phase velocities of the O and E rays of the incident fundamental wave are different due to the natural birefringence of the crystal. In general, the linear polarization of this input fundamental wave is turned into an elliptical polarization as it propagates through the crystal. The magnitude of the phase retardation between O and E rays is the product of the index dffference in the material and the effective optical path.
When such a Type II crystal is placed inside a laser resonator, this phase retardation can cause serious power loss when the laser is linearly polarized because the original linear polarization will not in general be properly maintained.
When the laser is randomly polarized, as is the case in multimode lasers when the laser active medium is not naturally birefringent and no polarizing elements are employed intracavity, the Type II SHG crystal provides a phase retardation between the polarization components resolved along its O and E axes. This retardation, which doubles on the return trip of the fundamental beam through the Type II SHG crystal can affect the stability and output power of the laser by affecting the laser's ability to optimize its polarization relative to thermal or other induced birefringent effects in the laser active medium. One can attempt to compensate this phase retardation using a passive device such as a Babinet-Soleil compensator. However, the retardation is usually dependent upon temperature and variations in temperature can be induced either by the ambient environment or by self-absorption of the laser radiation (fundamental and/or second harmonic) in the crystal itself. Such passive compensation thus becomes difficult to maintain during standard laser operation. Due to these problems, Type II SHG has typically been employed in an extracavity arrangement in which the polarization of the exiting fundamental wave from the SHG crystal is unimportant. Of course the advantage that the higher power density intracavity fundamental wave within the laser cavity has in generating second harmonic, is lost.
Many lasers can have the temporal form of their output power altered by a process known as Q-switching. Here, a special device which alters the optical quality or Q of the resonator is inserted into the beam within the resonator cavity. This "Q-switch" can be activated to produce enough optical loss to overcome the optical gain or amplification supplied by the laser active medium, thereby inhibiting oscillation. If the source exciting the laser active medium is maintained on during the low Q-period, energy is stored in the laser active medium in the form of an excess population inversion. When the Q-switch is turned off (returning the resonator quickly to its high Q state) this excess population is utilized to produce a high-intensity, Q-switched pulse. Since most Q-switches are electronically controlled, the process is repeatable at high repetition rates making a Q-switched laser a useful source of high intensity pulses. Peak pulse intensities many thousands of times the laser's continuous wave output power level can be generated. Because of the superior focusability and enhanced material interaction of shorter wavelengths, it is often of interest to frequency-double the output of Q-switched lasers.